This invention relates generally to methods and apparatus for reducing electromagnetic interferences (`EMI`) emissions in a digital system, and more particularly for reducing EMI emissions in a computer or other digitally-clocked system.
Electromagnetic interference is electromagnetic energy emitted from electronic devices which, either directly or indirectly, contributes to a degradation in performance of an electronic receiver or other electronic system. Poorly shielded electronic devices, for example, degrade radio and television signals resulting in audible or visible static at receivers picking up such signals. Governments typically regulate EMI emissions to enhance public use of the radio wave and other electromagnetic wave spectrums. In the United States, for example, the F.C.C. requires testing of devices and rates the devices by class according to their emissions. The United States F.C.C. Agency rates EMI emissions over a 120 kilohertz bandwidth. The 120 kHz bandwidth corresponds to the typical bandwidth of a conventional communication receiver, such as an FM receiver. Reduced EMI emissions within such bandwidth reduce the interference output perceived by a listener or viewer as, for example, static, white noise, or "ghosts."
Typical precautions taken by electronic manufacturers are to provide shielding of electronic devices to minimize EMI emissions. Computer manufacturers, for example, typically use shielded cables and shielded housings to minimize EMI emissions. This invention is directed toward a digital method and apparatus for spreading electromagnetic interference over a wide bandwidth so as to reduce the energy level within the 120 kHz bandwidth of commercial receivers.